The present invention relates to a sheath for light waveguides. The sheath comprises a member loosely surrounding one or more light waveguides. The sheath member absorbs axial tensile and compressive forces.
A series of problems arise in the manufacture of light waveguide cables when the synthetic resins and processing methods used in the manufacture of conventional cables are carried over into the manufacture of optical cables.
A fundamental difficulty in that the synthetic resins used for the coating, especially of the loose secondary coating, for the glass fibers and the sheath exhibit thermal behavior considerably different from that of light waveguide fibers. The thermal expansion differs by two orders of magnitude.
Moreover, synthetic resins, when subjected to tensile load, exhibit considerably larger expansion, which may be reversible or irreversible depending on the value of the tensile force.
Furthermore, extruded thermoplastics used in the manufacture of conventional telecommunication cables are subject to shrinkage due to aging. This shrinkage is accelerated when heated above the manufacturing temperature, insofar as it is not at least partly suppressed in a reeled cable.
When reeled, the cable and each of its components are subject to a bending strain. In the case of a synthetic resin tube, this also leads to a (preponderantly reversible) variation in length. When bent, the tube is longer than when straigtened.
These relationships have led to the definition of the term "overlength". According to cable construction materials and manufacturing processes, light waveguide fibers must be provided in the cable, especially in the secondary coating, with a certain overlength in order to obtain a symmetric "temperature-effective range" related to the manufacturing temperature. However, considerable problems arise in practice. In order to reduce these problems, experiments were performed to incorporate in the cable core and the cable sheath elements which absorb tensile and compressive forces. However, when incorporated in the core only, the behavior of the sheath is not influenced by it at all.
The incorporation of elements absorbing tensile and compressive forces in the sheath (for example laminating the sheath, closed armoring of twisted glass fiber connection elements, or mixed armoring of synthetic resin worms and glass fiber connection elements), however, also failed to provide satisfactory bending behavior. Whereas a bent pure synthetic resin tube is longer than a straight tube when the neutral bending line is shifted toward the center of curvature, shortening generally occurs when the known sheath constructions are bent and the neutral bending line is shifted away from the center of curvature. In the subsequent straightening, the shortening is reversible only partly or not at all.
In particular, bending of the sheath disrupts the connections between the materials so that the shrinkage behavior and the thermal expansion and contraction generally depend on whether the element was or was not previously bent.